1. Technical Field
The present disclosure relates a pouch-type secondary batter, and more particularly, to a pouch-type secondary battery exhibiting reduced swelling and corrosion of a cladding.
2. Description of the Related Art
As a portable electronic devices such as video cameras, portable phones, and portable computers become lighter while increasing performance, research is active on secondary batteries used as their power sources. Examples of suitable secondary batteries include nickel-cadmium batteries, nickel-hydride batteries, nickel-zinc batteries, and lithium secondary batteries.
Of these types of secondary batteries, lithium secondary batteries are widely used in many industrial fields because of small size, high capacity, high operating voltages, and excellent energy densities per unit weight. Lithium secondary batteries are classified according to the type of electrolyte as lithium ion batteries using a liquid electrolyte, and lithium polymer batteries using a polymer electrolyte.
Lithium secondary batteries are also classified according to the shape of the can into which the electrode assembly is inserted into cylinder-type, prism-type, and pouch-type batteries. A pouch-type secondary battery comprises a pouch cladding, which typically has a multi-layer structure comprising a metal foil layer and resin layers coated on top and bottom surfaces of the metal foil layer. Thus, the pouch-type is typically much lighter in weight than the cylindrical type or the prism type. Accordingly, it is possible to significantly reduce the weight of a secondary battery and to provide a variety of shapes using a pouch-type secondary battery.
A typical pouch-type secondary battery comprises an upper cladding and a lower cladding, formed, for example, by folding a rectangular cladding longitudinally. The lower cladding has a space formed therein by, for example, press processing. The space in the lower cladding accommodates an electrode assembly in a “jelly roll” form in which a sandwich of a positive electrode plate, a separator and a negative electrode plate is rolled-up. In a sealed pouch-type secondary battery, edges of the lower cladding are sealed to corresponding edges of the upper cladding are sealed to each other and the package pressurized.
Electrode tabs for providing an electrical connection with an external device are formed in the positive electrode plate and the negative electrode plate of the electrode assembly. The electrode tabs protrude from the electrode assembly axially relative to the winding axis of the jelly roll, and through a side of the sealed cladding. The electrode tabs are typically aluminum, copper, or nickel, with a thickness and size sufficient to serve as a current path without a large voltage drop.
In order to improve adhesion between a polymer layer inside the cladding and the electrode tab, the surface of the polymer layer may contain an adhesive, or an adhesive tape may be attached to the portion of the electrode tab that overlaps the cladding.
When thermally fusing the upper and lower claddings to each other, if the temperature and pressure are excessively high, or if fused too long, the polymer layer inside the cladding may be pushed out or get damaged, thereby causing a short circuit between the electrode tab and the metal foil of the cladding. If a short circuit occurs between the electrode tab and the metal foil, the battery discharges between the two electrode tabs through the metal foil, resulting in overheating and/or swelling, and thereby lowering the manufacturing yield. If a short circuit occurs between the negative electrode tab and the metal foil of the cladding, biasing the metal foil to a negative potential. At a metal corrosion potential of less than about 1 volt, corrosion of the metal foil occurs.
As a result, as a sealing capability of the cladding is degraded, an organic electrolyte in the electrode assembly may be evaporated, and external humidity and oxygen penetrates the cladding, generating gas therein causing swelling of the cladding.